Bumping gaia.json for 2 gaia-central revision(s)

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https://hg.mozilla.org/integration/gaia-central/rev/6d539d255cad
Author: Kevin Grandon <kevingrandon@yahoo.com>
Desc: Merge pull request #11470 from hotzenklotz/903521_camera_in_FF

Bug 903521 - Camera not working in gaia when run in Firefox r=kgrandon

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https://hg.mozilla.org/integration/gaia-central/rev/f2b2ca763a75
Author: Tom Herold <therold@mozilla.com>
Desc: Bug 903521 - Camera not working in gaia when run in Firefox

b2g/config/gaia.json

@@ -1,4 +1,4 @@
 {
-    "revision": "0e3e30e489ff38cceb8b9cf9ee5caea5fb072457", 
+    "revision": "6d539d255cadff4ef4b1982b29d6479c4b736b40", 
     "repo_path": "/integration/gaia-central"
 }